Implement posix_spawn

Since Redox's execv* and fork implementations are based on the lower-level kernel context handles, it would be preferable to provide a custom implementation of posix_spawn{,p}, using those native APIs. The API in redox-exec already supports executing a remote process, which could be combined with the fork API, minus the copy-everything-and-then-reinitialize-everything parts.

There are also strong reasons to perhaps deprecate fork entirely, and is the reason fork has not been added to libredox yet, and may never be.